Level shifter and a method for shifting voltage level

ABSTRACT

A level shifter comprises a first control switch ( 207 ) for connecting an output terminal to a first supply voltage (VDDH) to set an output signal to be high, and a second control switch ( 208 ) for connecting the output terminal to a signal ground (GND) to set the output signal to be low. The level shifter comprises a pre-charging switch ( 210 ) for connecting the output terminal to the first supply voltage, and an input gate circuit ( 211 ) for controlling an ability of an input signal to control the second control switch. The level shifter comprises a keeper circuit ( 212 ) for controlling the first control switch based on the output signal. The first control switch is controlled with the first supply voltage when the output signal is low, and with a second supply voltage that is between the first supply voltage and the signal ground when the output signal is high.

FIELD OF THE DISCLOSURE

The disclosure relates to a level shifter for producing an output signalbased on an input signal. The disclosure relates also to an electronicdevice comprising one or more level shifters. Furthermore, thedisclosure relates to a method for controlling a level shifter.

BACKGROUND

The energy consumption of an electronic device, such as a digitalprocessor, is a critical issue in many cases. For example, technologiessuch as the Internet-of-Things “IoT”, the Industrial Internet “II”, andthe Internet-of-Everything “IoE” are on the threshold of a massivebreakthrough, and the major drivers behind the break-through areubiquitous wireless processing nodes. However, the energy consumption oftransmitting a bit across a given distance does not scale with Moore'slaw as advantageously as the digital processing within a wireless node.Therefore, the energy cost of wireless transmission will proportionallygrow when compared to digital processing. Increasing the energyefficiency thus requires increasing the amount of intra-node processingin order to minimize the wireless transmission of data. Therefore, theprocessor and the digital signal processing “DSP” will become one ofthe, if not the, most important parts to be optimized. This will becompounded by the increasing functionalities of the wireless node, suchas e.g. Machine Learning, Video, etc.

Increasing the energy efficiency may result in a design where differentsections of an electronic device operate with different supply voltages.For example, in ultra-low-power processor scenarios, memory blocks mayrequire supply voltages that are higher than supply voltages ofprocessors. Thus, the memory blocks belong to a higher voltage domain ofthe electronic device whereas the processors belong to a lower voltagedomain of the electronic device. In some cases and especially inconjunction with central processing units “CPU”, a supply voltage of aprocessor is often called core voltage.

One or more level shifters is/are typically required in order totransfer digital data from a lower voltage domain operating at a lowervoltage to a higher voltage domain operating at a higher voltage. Theenergy consumption of the level shifters needs to be minimized and isespecially important for example in cases where wide data buses areunder the need for level shifting.

A known approach to reduce energy consumption of a level shifter is touse pre-charging where the level shifter is pre-charged to the voltageof a higher voltage domain prior to transferring digital data from alower voltage domain to the higher voltage domain. The pre-charging istypically called a pre-charging phase and the transfer of the digitaldata is typically called an evaluation phase. Level shifters where thepre-charging is used are presented e.g. in publications U.S. Pat. No.8,860,488 and US20150207506. In the level shifters described in U.S.Pat. No. 8,860,488 and US20150207506, two cross coupled p-channel metaloxide semiconductor “PMOS” transistors are utilized and thus twoseparate discharging branches are needed. Publication WO2011097628describes a level shifter where only one pre-charged branch and only onedischarging path are needed and where the pre-charged output terminal ofthe level shifter is kept at a high logical value with the aid of akeeper circuit. The keeper circuit, however, needs gating informationfrom another level shifter with a delay, and this arrangement increasesthe energy consumption as well as the layout area.

SUMMARY

The following presents a simplified summary in order to provide basicunderstanding of some aspects of various invention embodiments. Thesummary is not an extensive overview of the invention. It is neitherintended to identify key or critical elements of the invention nor todelineate the scope of the invention. The following summary merelypresents some concepts in a simplified form as a prelude to a moredetailed description of exemplifying embodiments of the invention.

In accordance with the invention, there is provided a new level shifterfor producing an output signal based on an input signal. A level shifteraccording to the invention comprises:

-   -   an input terminal for receiving the input signal and an output        terminal for outputting the output signal,    -   a level shift circuit comprising a first control switch for        connecting the output terminal to a first supply voltage so as        to set the output signal to a first logical value and a second        control switch for connecting the output terminal to a signal        ground so as to set the output signal to a second logical value,    -   a pre-charge circuit comprising a pre-charging switch for        connecting the output terminal to the first supply voltage and        an input gate circuit for controlling the ability of the input        signal to control the second control switch, and    -   a keeper circuit responsive to the output signal and for        connecting the first supply voltage to the control terminal of        the first control switch so as to keep the first control switch        non-conductive when the output signal has the second logical        value, and for connecting a second supply voltage to the control        terminal of the first control switch so as to keep the first        control switch conductive when the output signal has the first        logical value.

During a pre-charging phase, the output terminal is charged via thepre-charging switch to the first supply voltage and the input gatecircuit is controlled to disable the input signal from controlling thesecond control switch. During a subsequent evaluation phase, thepre-charging switch is non-conductive and the input gate circuit enablesthe input signal to control the second control switch.

As mentioned above, the control terminal of the first control switch isconnected to the second supply voltage when the output signal has itsfirst logical value. With suitable values of the second supply voltage,the ability of the first control switch to keep the output signal at thefirst logical value is sufficiently weak so that the second controlswitch is sufficiently strong to pull the output signal to the secondlogical value when the input signal corresponds to the second logicalvalue and controls the second control switch. It is also possible toselect the electrical properties of the first and second controlswitches, e.g. channel impedances in a case of metal oxide semiconductor“MOS” field-effect transistors, so that the ability of the first controlswitch to keep the output signal at the first logical value whencontrolled by the second supply voltage is sufficiently weaker than theability of the second control switch to pull the output signal to thesecond logical value when the input signal corresponds to the secondlogical value and controls the second control switch.

In the above-described level shifter, there is no need for cross coupledswitches and for two separate discharging branches. Furthermore, forbeing able to operate, the above-described level shifter does not needgating information from another level shifter.

In accordance with the invention, there is provided also a new methodfor controlling a level shifter of the kind described above. A methodaccording to the invention comprises:

-   -   connecting the first supply voltage to the control terminal of        the first control switch so as to keep the first control switch        non-conductive when the output signal has the second logical        value, and    -   connecting the second supply voltage to the control terminal of        the first control switch so as to keep the first control switch        conductive when the output signal has the first logical value,        the electric potential of the second supply voltage being        between those of first supply voltage and the signal ground.

In accordance with the invention, there is provided also a newelectronic device that comprises

-   -   a first functional section and a second functional section,    -   one or more level shifters according to the invention and        configured to receive one or more input signals from the first        functional section, produce one or more output signals based on        the one or more input signals, and supply the one or more output        signals to the second functional section, and    -   a voltage supply circuit configured to produce the first supply        voltage and the second supply voltage and supply the first and        second supply voltages to the one or more level shifters, the        electric potential of the second supply voltage being between        those of first supply voltage and the signal ground.

The supply voltage of first functional section can be for example thesecond supply voltage, and the supply voltage of second functionalsection can be for example the first supply voltage. The firstfunctional section may comprise for example one or more digitalprocessors and the second functional section may comprise for exampleone or more memory circuits.

A number of exemplifying and non-limiting embodiments of the inventionare described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention bothas to constructions and to methods of operation, together withadditional objects and advantages thereof, will be best understood fromthe following description of specific exemplifying embodiments when readin connection with the accompanying drawings.

The verbs “to comprise” and “to include” are used in this document asopen limitations that neither exclude nor require the existence of alsoun-recited features.

The features recited in the accompanied dependent claims are mutuallyfreely combinable unless otherwise explicitly stated. Furthermore, it isto be understood that the use of “a” or “an”, i.e. a singular form,throughout this document does not exclude a plurality.

BRIEF DESCRIPTION OF THE FIGURES

Exemplifying and non-limiting embodiments of the invention and theiradvantages are explained in greater detail below with reference to theaccompanying drawings, in which:

FIG. 1 shows a high-level block diagram of an electronic devicecomprising level shifters according to an exemplifying and non-limitingembodiment of the invention,

FIG. 2 shows a circuit diagram of a level shifter according to anexemplifying and non-limiting embodiment of the invention,

FIG. 3 illustrates the operation of the level shifter shown in FIG. 2,and

FIG. 4 shows a flowchart of a method according to an exemplifying andnon-limiting embodiment of the invention for controlling a levelshifter.

DESCRIPTION OF EXEMPLIFYING AND NON-LIMITING EMBODIMENTS

The specific examples provided in the description below should not beconstrued as limiting the scope and/or the applicability of theaccompanied claims. Lists and groups of examples provided in thedescription below are not exhaustive unless otherwise explicitly stated.

FIG. 1 shows a high-level block diagram of an electronic device thatcomprises level shifters according to an exemplifying and non-limitingembodiment of the invention. Three of the level shifters are denotedwith references 101, 102 and 103. The electronic device comprises afirst functional section 120 and a second functional section 121. Thelevel shifters are configured to receive input signals IN(1) . . . IN(N)from the first functional section 120. The level shifters are configuredproduce output signals OUT(1) . . . OUT(N) based on the input signals,and supply the output signals to the second functional section 121. Inthis exemplifying case, the electronic device comprises a voltage supplycircuit 122 configured to produce a first supply voltage VDDH and asecond supply voltage VDDL so that the electric potential of the secondsupply voltage VDDL is between those of first supply voltage VDDH and asignal ground GND. Typically, the electric potential of the signalground GND is deemed to be zero. The first functional section 120 maycomprise for example one or more digital processors whose supply voltageis the second supply voltage VDDL, and the second functional section 121may comprise for example one or more memory circuits whose supplyvoltage is the first supply voltage VDDH.

During a pre-charging phase, the input signals IN(1) . . . IN(N) aredisabled from controlling the output signals OUT(1) . . . OUT(N) and theoutput terminals of the level shifters are pre-charged to the firstsupply voltage VDDH. In this exemplifying case, it is assumed that thefirst supply voltage VDDH is positive with respect to the signal groundGND. Thus, the first supply voltage VDDH represents the higher logicalvalue of the output signals OUT(1) . . . OUT(N) and the signal groundGND represents the lower logical value of the output signals. During anevaluation phase subsequent to the pre-charging phase, each input signalwhich has its lower logical value controls the respective one of theoutput signals to its lower logical value. The lower logical value ofthe input signals IN(1) . . . IN(N) can be the signal ground GND, andthe higher logical value of the input signals can be the second supplyvoltage VDDL. The second functional section 121 can be configured tocontrol the above-mentioned pre-charging and evaluation phases forexample so that a first pre-charging control signal PRE disables theinput signals IN(1) . . . IN(N) from controlling the output signalsOUT(1) . . . OUT(N) when the first pre-charging control signal PRE hasits higher logical value, and a second pre-charging control signal PREBcontrols the level shifters to pre-charge their output terminals to thefirst supply voltage VDDH when the second pre-charging control signalPREB has its lower logical value. The evaluation phase begins when thefirst pre-charging control signal PRE has been changed to its lowerlogical value and the second pre-charging control signal PREB has beenchanged to its higher logical value. Advantageously, the first andsecond pre-charging control signals PRE and PREB are changedsimultaneously or the second pre-charging control signal PREB is changedprior to changing the first pre-charging control signal PRE, i.e. theinput signals IN(1) . . . IN(N) are not enabled to control the outputsignals OUT(1) . . . OUT(N) when the pre-charging takes place. Thehigher logical values of the first and second pre-charging controlsignals PRE and PREB can be e.g. the first supply voltage VDDH, and thelower logical values of the first and second pre-charging controlsignals can be the signal ground GND. Typically, the above-describedoperation where the second functional section 121 controls thepre-charging control signals PRE and PREB requires synchronizationbetween the functional sections 120 and 121 so that timing informationis delivered from the functional section 120 to the functional section121. This synchronization can be implemented with suitable known ways.It is also possible that the first pre-charging control signal PRE comesfrom the first functional section 120 instead of the second functionalsection 121. In both cases, the pre-charging control signals PRE andPREB need to be synchronized with each other in order to avoid asituation where pull-up and pull-down paths of the level shifters aresimultaneously in the conductive state.

FIG. 2 shows a circuit diagram of the level shifter 101 shown in FIG. 1.The other level shifters shown in FIG. 1 can be similar to the levelshifter 101. The level shifter 101 comprises an input terminal 204 forreceiving the input signal IN and an output terminal 205 for outputtingthe output signal OUT. The level shifter 101 comprises a level shiftcircuit 206, a pre-charge circuit 209, and a keeper circuit 212. Thelevel shift circuit 206 comprises a first control switch 207 forconnecting the output terminal 205 to the first supply voltage VDDH soas to set the output signal OUT to its first logical value and a secondcontrol switch 208 for connecting the output terminal 205 to the signalground GND so as to set the output signal OUT to its second logicalvalue. In this exemplifying case, it is assumed that the first supplyvoltage VDDH that represents the first logical value of the outputsignal is positive with respect to the signal ground GND that representsthe second logical value of the output signal. Thus, the first logicalvalue of the output signal OUT is the higher logical value of the outputsignal and the second logical value of the output signal OUT is thelower logical value of the output signal. The first control switch 207can be for example a p-channel metal oxide semiconductor field-effecttransistor “PMOS”, and the second control switch 208 can be for examplean n-channel metal oxide semiconductor field-effect transistor “NMOS”.

The pre-charge circuit 209 comprises a pre-charging switch 210 forconnecting the output terminal to the first supply voltage VDDH. Thepre-charging switch 210 is controlled with the aid of the secondpre-charging control signal PREB. The pre-charging switch 210 can be forexample a p-channel metal oxide semiconductor field-effect transistor“PMOS”. In this exemplifying case, the pre-charging switch 210 isconductive when the second pre-charging control signal PREB has itslower logical value and non-conductive when the second pre-chargingcontrol signal PREB has its higher logical value. The pre-charge circuit209 further comprises an input gate circuit 211 for controlling theability of the input signal IN to control the second control switch 208.In this exemplifying case, the input gate circuit 211 comprises aninverting OR-gate, i.e. a NOR-gate for forming the NOR-function of theinput signal IN and the first pre-charge control signal PRE. The outputof the NOR-gate is connected to the control terminal of the secondcontrol switch 208. When the first pre-charge control signal PRE has itshigher logical value, the out-put of the NOR-gate is the signal groundGND irrespective of the input signal IN. When the first pre-chargecontrol signal PRE has its lower logical value and the input signal INhas its higher logical value, the output of the NOR-gate is the signalground GND. When the first pre-charge control signal PRE has its lowerlogical value and the input signal IN has its lower logical value, theoutput of the NOR-gate is the second supply voltage VDDL. The secondcontrol switch 208 is conductive when the output of the NOR-gate is thesecond supply voltage VDDL, and the second control switch 208 isnon-conductive when the output of the NOR-gate is the signal ground GND.

The keeper circuit 212 is responsive to the output signal OUT andconnects the first supply voltage VDDH to the control terminal 213 ofthe first control switch 207 so as to keep the first control switch 207non-conductive when the output signal OUT has its lower logical value,i.e. the signal ground GND. The keeper circuit 212 connects the secondsupply voltage VDDL to the control terminal 213 of the first controlswitch 207 so as to keep the first control switch 207 conductive whenthe output signal OUT has the higher logical value, i.e. the firstsupply voltage VDDH. In this exemplifying case, the keeper circuit 212comprises an inverter whose input is configured to receive the outputsignal OUT and whose output is connected to the control terminal 213 ofthe first control switch. The higher output value of the inverter is thefirst supply voltage VDDH and the lower output value of the inverter isthe second supply voltage VDDL.

A level shifter according to an exemplifying and non-limiting embodimentof the invention comprises a voltage supply circuit 222 configured toproduce the first supply voltage VDDH and the second supply voltage VDDLso that the electric potential of the second supply voltage VDDL isbetween those of first supply voltage VDDH and the signal ground GND.The voltage supply circuit 222 may comprise for example linearregulators for controlling the VDDH and VDDL. As illustrated in FIG. 1,it is however also possible that a level shifter according to anexemplifying and non-limiting embodiment of the invention receives thefirst and second supply voltages VDDH and VDDL from a source that isexternal to the level shifter.

FIG. 3 presents exemplifying cases which illustrate the operation of thelevel shifter 101. In the first exemplifying case, a pre-charging phasetakes place between time instants t1 and t2. During the pre-chargingphase, the first pre-charge control signal PRE has its higher logicalvalue and the second pre-charge control signal PREB has its lowerlogical value. The pre-charging switch 210 is conductive and the inputgate circuit 211 keeps the second control switch 208 non-conductive. Itis assumed that the input signal IN changes from its lower logical valueto its higher logical value during the pre-charging phase. This changeof the input signal IN has, however, no effect during the pre-chargingphase. The pre-charging phase ends and the subsequent evaluation phasebegins at the time instant t2 when the second pre-charge control signalPREB is changed to its higher logical value and the first pre-chargecontrol signal PRE is changed to its lower logical value. The change ofthe PREB to its higher logical value turns the pre-charging switch 210to the non-conductive state and thereafter only the first control switch207 which is controlled by the second supply voltage VDDL is keeping theoutput signal OUT at its higher logical value. The change of the PRE toits lower logical value enables the input signal IN to control thesecond control switch 208. As the input signal IN has its higher logicalvalue during the evaluation phase, the second control switch 208 staysnon-conductive and thus the output signal stays at its higher logicalvalue.

In the second exemplifying case illustrated in FIG. 3, a pre-chargingphase takes place between time instants t3 and t4. During thepre-charging phase, the first pre-charge control signal PRE has itshigher logical value and the second pre-charge control signal PREB hasits lower logical value. The pre-charging switch 210 is conductive andthe input gate circuit 211 keeps the second control switch 208non-conductive. It is assumed that the input signal IN changes from itshigher logical value to its lower logical value during the pre-chargingphase. This change of the input signal IN has, however, no effect duringthe pre-charging phase. The pre-charging phase ends and the subsequentevaluation phase begins at the time instant t4 when the secondpre-charge control signal PREB is changed to its higher logical valueand the first pre-charge control signal PRE is changed to its lowerlogical value. The change of the PREB to its higher logical value turnsthe pre-charging switch 210 to the non-conductive state and thereafteronly the first control switch 207 which is controlled by the secondsupply voltage VDDL tends to keep the output signal OUT at its higherlogical value. The change of the PRE to its lower logical value enablesthe input signal IN to control the second control switch 208. As theinput signal IN has its lower logical value during the evaluation phase,the second supply voltage VDDL is conducted to the control terminal ofthe second control switch 208 and thus the second control switch 208becomes conductive. The control terminal 213 of first control switch 207is connected to the second supply voltage VDDL when the output signalOUT has its higher logical value. With suitable values of the secondsupply voltage VDDL, the ability of the first control switch 207 to keepthe output signal OUT at its higher logical value is so weak that thesecond control switch 208 is sufficiently strong to pull the outputsignal OUT to its lower logical value when the input signal IN has itslower logical value and is enabled to control the second control switch208. It is also possible to select the electrical properties of thefirst and second control switches 207 and 208, e.g. channel impedancesof MOS transistors, so that the ability of the first control switch 207to keep the output signal OUT at its higher logical value whencontrolled by the second supply voltage VDDL is sufficiently weaker thanthe ability of the second control switch 208 to pull the output signalOUT to its lower logical value when the input signal IN has its lowerlogical value and is enabled to control the second control switch 208.After the second control switch 208 has pulled the out-put signal OUT toits lower logical value, the keeper circuit 212 connects the firstsupply voltage VDDH to the control terminal 213 of the first controlswitch 207 and thus the first control switch 207 becomes non-conductive.

FIG. 4 shows a flowchart of a method according to an exemplifying andnon-limiting embodiment of the invention for controlling a level shifterthat comprises:

-   -   an input terminal for receiving an input signal IN and an output        terminal for outputting an output signal OUT,    -   a level shift circuit comprising a first control switch for        connecting the output terminal to a first supply voltage VDDH so        as to set the output signal OUT to a first logical value and a        second control switch for connecting the output terminal to a        signal ground GND so as to set the output signal OUT to a second        logical value, and    -   a pre-charge circuit comprising a pre-charging switch for        connecting the output terminal to the first supply voltage VDDH        and an input gate circuit for controlling the ability of the        input signal IN to control the second control switch.

The method comprises:

-   -   action 401: connecting the first supply voltage VDDH to the        control terminal of the first control switch that is between the        output terminal and the first supply voltage VDDH so as to keep        the first control switch non-conductive when the output signal        OUT has the second logical value, and    -   action 402: connecting a second supply voltage VDDL to the        control terminal of the first control switch so as to keep the        first control switch conductive when the output signal OUT has        the first logical value, the electric potential of the second        supply voltage VDDL being between those of first supply voltage        VDDH and the signal ground GND.

In a method according to an exemplifying and non-limiting embodiment ofthe invention, the first control switch is controlled with an inverterwhose input receives the output signal OUT and whose output is connectedto the control terminal of the above-mentioned first control switch. Theoutput of the inverter is connected to the first supply voltage VDDHwhen the output signal OUT has the second logical value and the outputof the inverter is connected to the second supply voltage VDDL when theoutput signal OUT has the first logical value.

The specific examples provided in the description given above should notbe construed as limiting the scope and/or the applicability of theappended claims. Lists and groups of examples provided in thedescription given above are not exhaustive unless otherwise explicitlystated.

1. A level shifter for producing an output signal based on an inputsignal, the level shifter comprising: an input terminal for receivingthe input signal and an output terminal for outputting the outputsignal, a level shift circuit comprising a first control switch forconnecting the output terminal to a first supply voltage so as to setthe output signal to a first logical value and a second control switchfor connecting the output terminal to a signal ground so as to set theoutput signal to a second logical value, a pre-charge circuit comprisinga pre-charging switch for connecting the output terminal to the firstsupply voltage and an input gate circuit for controlling an ability ofthe input signal to control the second control switch, and a keepercircuit responsive to the output signal and for connecting the firstsupply voltage to a control terminal of the first control switch so asto keep the first control switch non-conductive when the output signalhas the second logical value, wherein the keeper circuit is configuredto connect a second supply voltage to the control terminal of the firstcontrol switch so as to keep the first control switch conductive whenthe output signal has the first logical value.
 2. A level shifteraccording to claim 1, wherein the first control switch is a p-channelmetal oxide semiconductor field-effect transistor and the second controlswitch is an n-channel metal oxide semiconductor field-effecttransistor.
 3. A level shifter according to claim 1, wherein thepre-charging switch is a p-channel metal oxide semiconductorfield-effect transistor.
 4. A level shifter according to claim 1,wherein, the input gate circuit comprises a NOR-gate for forming aNOR-function of the input signal and a pre-charge control signal and forsupplying the NOR-function to a control terminal of the second controlswitch.
 5. A level shifter according to claim 1, wherein the keepercircuit comprises an inverter whose input is configured to receive theoutput signal and whose output is connected to the control terminal ofthe first control switch, the output of the inverter being connected tothe first supply voltage when the output signal has the second logicalvalue, and the output of the inverter being connected to the secondsupply voltage when the output signal has the first logical value.
 6. Alevel shifter according to claim 1, wherein the level shifter comprisesa voltage supply circuit configured to produce the first supply voltageand the second supply voltage so that an electric potential of thesecond supply voltage is between those of first supply voltage and thesignal ground.
 7. An electronic device comprising: a first functionalsection and a second functional section, characterized in furthercomprising: one or more level shifters according to claim 1, andconfigured to receive one or more input signals from the firstfunctional section, produce one or more output signals based on the oneor more input signals, and supply the one or more output signals to thesecond functional section, and a voltage supply circuit configured toproduce the first supply voltage and the second supply voltage andsupply the first and second supply voltages to the one or more levelshifters, an electric potential of the second supply voltage beingbetween those of first supply voltage and the signal ground.
 8. A methodfor controlling a level shifter that comprises: an input terminal forreceiving an input signal and an output terminal for outputting anoutput signal, a level shift circuit comprising a first control switchfor connecting the output terminal to a first supply voltage so as toset the output signal to a first logical value and a second controlswitch for connecting the output terminal to a signal ground so as toset the output signal to a second logical value, and a pre-chargecircuit comprising a pre-charging switch for connecting the outputterminal to the first supply voltage and an input gate circuit forcontrolling an ability of the input signal to control the second controlswitch, the method comprising: connecting the first supply voltage to acontrol terminal of the first control switch so as to keep the firstcontrol switch non-conductive when the output signal has the secondlogical value, wherein the method comprises connecting a second supplyvoltage to the control terminal of the first control switch so as tokeep the first control switch conductive when the output signal has thefirst logical value, an electric potential of the second supply voltagebeing between those of first supply voltage and the signal ground.
 9. Amethod according to claim 8, wherein the first control switch iscontrolled with an inverter whose input receives the output signal andwhose output is connected to the control terminal of the first controlswitch, the output of the inverter being connected to the first supplyvoltage when the output signal has the second logical value, and theoutput of the inverter being connected to the second supply voltage whenthe output signal has the first logical value.